CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Statement of Comprehensive Income [Abstract]
Net income	$ 416.8	$ 665.7	$ 737.3
Pension Plan and Restoration of Retirement Income Plan:
Amortization of prior service cost, net of tax of $0.1, $0.1 and $0.0, respectively	0.2	0.2	0.1
Amortization of deferred net loss, net of tax of $0.2, $0.2 and $0.9, respectively	0.9	1.4	1.6
Net gain (loss) arising during the period, net of tax of $1.0, ($2.4) and $0.0, respectively	3.3	(7.6)	1.4
Prior service cost arising during the period, net of tax of $0.0, $0.0 and ($0.3), respectively	0.0	0.0	(1.1)
Settlement cost, net of tax of $0.4, $4.3 and $2.7, respectively	1.2	13.6	6.0
Postretirement benefit plans:
Amortization of prior service credit, net of tax of $0.0, ($0.1) and ($0.4), respectively	0.0	(0.2)	(1.4)
Amortization of deferred net (gain) loss, net of tax of ($0.1), $0.0 and $0.0, respectively	(0.1)	0.0	0.1
Net gain (loss) arising during the period, net of tax of ($0.2), $1.7 and ($0.2), respectively	(0.8)	5.5	(0.7)
Other comprehensive gain from unconsolidated affiliates, net of tax $0.0, $0.0 and $0.3, respectively	0.0	0.0	1.3
Other comprehensive income, net of tax	4.7	12.9	7.3
Comprehensive income	$ 421.5	$ 678.6	$ 744.6